Falls and fall related injuries are among the most serious, common and costly medical problems facing the elderly. There are about 300,000 hip fractures annually in the U.S., over 90% of them associated with falls. Noting that no previous study had examined the role of fall mechanics in the etiology of hip fracture, during the previous funding period, the configurations and velocities of fallers just before hip impact was determined. That information was used to estimate impact force, and to develop analytical models for interpreting and extending these experimental results to the frail elderly. The applicant showed, in these studies, that impact velocity is most sensitive to body height and impact force to body weight, that active and passive energy absorption in the joints and muscles of the lower extremity during falling absorb roughly 71% of the potential energy available and that falls to the side are dangerous in part because the hip contacts the ground first, even in young volunteers instructed to break a self-initiated "fall" with the outstretched hand. The applicant has also shown that impact forces can double with activation of the trunk muscles, that a 3-fold increase in trochanteric soft tissue thickness reduces impact force by about one-third, and that impact forces for an average fall exceed the strength of the elderly hip by about 35%. Recently, the applicant, and others, have further shown from surveillance studies that falls to the side, particularly those that end with impact on the hip, raise the risk of hip fracture from 6- to 30-fold, compared to about 3-fold increases in risk associated with one standard deviation reduction in bone mineral density at the hip. And yet, despite the apparently crucial etiologic importance of falling to the side, previous research on postural stability has largely focused on motion in the plane of progression. In addition, while previous studies have helped identify those factors that place the elderly at high risk for falling, no previous work has identified subjects at increased risk of falling to the side or examined the effects of asymmetries in lower extremity strength, reaction time, or musculoskeletal pathology as potential risk factors for sideways falls. Given the dramatically increased risk of hip fracture from a fall to the side, Aim 1 is to understand the fundamental dynamics of falling to the side. Since fall prevention efforts have not as yet demonstrated reductions in the incidence of injurious falls and fall risk assessments cannot be used to identify the small fraction of fallers who will suffer a hip fracture, Aim 2 is to identify elderly subjects at high risk for sideways falls. And finally, since hip fracture can be prevented by reducing the severity of those falls that do occur, Aim 3 proposes to explore two new intervention strategies, one focused on reducing impact velocity and the other on impact force.